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dumpvalve on a tdi

Discussion in 'A3/S3/Sportback (8P Chassis)' started by evocos, Aug 14, 2009.

  1. evocos
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    evocos Member

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    [Aug 14, 2009]
    I've got the chance of a dumpvalve i could fit to my 140 chipped tdi,is there any benefit in having one ?
    I would also fit a switch so i could turn it on and off,or is this a bit chavtastic,i always liked the sound from my escort cossie days
    Last edited by a moderator: Aug 16, 2009
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  3. devonmikeyboy
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    devonmikeyboy As far from JBS as possible !

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    [Aug 14, 2009]
    Petrol engines have a throttle mechanism which restricts the flow of air between inlet (turbo-compressor) and the engine combustion chamber, necessary to keep the air-fuel mixture in the ignitable range while controlling the engine's power output. Both mass of air taken in and fuel added must be carefully controlled.

    When you close the throttle on such an engine the compressor is still pushing air into the inlet and it suddenly has nowhere to go. The overpressure could deform a throttle butterfly, and will inevitably slow the turbo down due to back pressure (thus also putting more back pressure on the exhaust), leading to turbo lag when the throttle is opened again. The solution is to provide a pressure-actuated valve, which reacts to an excessive difference in pressure between the two sides of the throttle, opening and dumping the excess pressure to the ambient (dump valve) or back into the inlet (recirculating blowoff, which does a much better job by equalising the pressure on both sides of the turbo). Without this, throttles and turbos would have to be much more heavily engineered and throttle response would suffer.

    So dump valves are about throttle response at gearchange and other sudden transient events, not about performance in the leadfoot top-speed way of thinking.

    The need for a dump valve is a weakness of turbocharged petrol engines, putting another contraption in the inlet side which disturbs the gas flow and is a point of failure. I find it mildly astonishing that anyone should be proud of their car farting on the overrun. But then I run a diesel so I'm probably disqualified from having an opinion.[​IMG]

    Diesel engines have no throttle. Power output is controlled by the fuel quantity injected at each stroke. Gas flow around the compressor-combustion chamber-turbo impeller loop is always uniform. This makes diesels much better candidates for forced induction.

    A dump valve can only do harm to the performance of a diesel. Turbo wastegates are not dump valves, they limit the pressure on the impeller (exhaust) side of the turbine to prevent the turbine from overspeeding when the engine's gas flow becomes too high i.e. at high rev's. The higher-performing VAG turbos use variable-geometry inlet vanes and have no wastegate.
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  4. D3mon
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    D3mon 2009 A3 2.0 TDI Black Edition

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    [Aug 15, 2009]
    Now that is a fine post. Thanks for taking the time! :D
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  5. mhill
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    mhill Member VCDS Map User

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    [Aug 15, 2009]
    Wow nice write up a description.

    I guess the OP is referring to the Forge TDI dump valve which i understand is a over engineered speaker though i may be wrong.
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  6. Broken Byzan
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    Broken Byzan Photographic Moderator Staff Member Moderator VCDS Map User

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    [Aug 15, 2009]
    Slight correction here, diesel engines do have a throttle as they are the same as any other engine. Its the amount of air you can get into an engine that restricts the power, the fuel is injected at a specific ratio to promote the most efficient burn.
    If it wasn't air that drove the power levels, then a turbo simply would not work to boost power levels
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  7. mhill
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    mhill Member VCDS Map User

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    [Aug 15, 2009]
    They dont have a throttle. In a simple way its ram more fuel in, engine runs faster, sucks more air in spinning turbo faster.
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  8. mhill
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    mhill Member VCDS Map User

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    [Aug 15, 2009]
    Sorry they dont!
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  9. devonmikeyboy
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    devonmikeyboy As far from JBS as possible !

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    [Aug 15, 2009]
    The Forge dump valve is no longer made but at least it used a proper dump valve. Cheaper kits like the Sabre etc are just a speaker you fit in the engine bay controlled by a switch.
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  10. evocos
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    evocos Member

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    [Aug 15, 2009]
    Thankyou very much for the very thorough explanation,and very well written,but sorry i do pretty much understand the workings of one,having owned 6 cossy's,rs turbos,evo's etc,but having one on a diesel seems weird.I think that the one my mate has fitted is pretty much for effect only (as mentioned by mhill)i know it was about £200 squid but,i'm not sure how they work.
    I just thought it would be cool,maybe i should remember i'm getting older :lmfao:but as mentioned it would have an off switch.
    If anyone knows what the hell i'm on about would they know if there is a downside to one being fitted.
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  11. mikep
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    mikep Member

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    [Aug 16, 2009]
    A dumpvalve on a diesel is the stupidest thing in the history of stupid ideas IMHO

    A dumpvalve on a VAG petrol turbo is also a very stupid idea as they are designed to use a diverter valve.

    Happy Sunday!
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